Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Light Red Spinel, Burma
3.15ct., Weak, SI 17
Light Pink Spinel, Sri Lanka
2.26ct., Weak, SI 17
Light Pink Spinel, Sri Lanka
3.67ct., Diamagnetic, SI < 0)
Vivid Pink Spinel, Tajikistan
4.49ct., Weak, SI <20
Purple Spinel, Sri Lanka
1.71ct, Weak, SI 56
Purple Spinel, Sri Lanka
3.46ct, Weak, SI 30
Pale Violet Spinel, Sri Lanka
2.15ct., Weak, SI 30
Near-Colorless Pink Spinel
1.70ct., Diamagnetic, SI <0
Near-Colorless Gray Spinel
3.07ct., Weak, SI 17
Bluish Gray Spinel
1.49ct., Weak, SI 49
Grayish Green Spinel
1.94ct., Weak, SI 42
Light Blue Synthetic Spinel
1.45ct., Diamagnetic, SI <0
Dark Blue Synthetic Spinel
1.33ct, Diamagnetic, SI <0
Blue Synthetic Spinel with Manganese
18.74ct., Weak, SI 39
Synthetic Pink Spinel
1.05ct., Diamagnetic, SI <0
Synthetic Red Spinel
7.38ct., Weak, SI <20
Dark Red Spinel, Sri Lanka
3.72ct., Weak, SI 22
The two dark red Spinels shown below have similar color, but one is much more magnetic than the other. The gem on the left is weakly magnetic, while the gem on the right is the only red Spinel we tested that showed a strong magnetic response to a handheld magnet (SI 139).
Chromium is responsible for color in red, pink, and orange Spinels. The concentration of chromium in some vivid red Spinels can be high enough to contribute to the total measurable magnetism. All red Spinels in this study showed detectable magnetism. Most were only weakly magnetic.
Iron rather than chromium is likely the main contributor to magnetism in dark red Spinels. As illustrated below, the light red gem on the left has similar magnetic susceptibility as the dark red gem on the right. The lighter red gem has higher chromium content, and it's magnetic susceptibility is perhaps due primarily to chromium. It fluoresces under long wave UV light. The dark red gem on the right does not fluoresce, and the magnetism is likely due primarily to iron.
4.96ct., Strong, SI 139
The reasons for such variations in intensity of red color, and for the irregular association between color intensity and magnetic susceptibility, are not clear. Variation may be related to the amount of chromium and to different ratios of ferrous iron (Fe2+) to ferric iron (Fe3+). Lower chromium content in conjunction with gray color due to a small amount of ferrous iron (Fe2+) might result in dark red color and low magnetic susceptibility, as in the red oval gem above left. More ferric iron (Fe3+) might induce higher magnetic susceptibility without darkening color, as in the red rectangular gem above right.
Pink Spinel: Chromium is present only in trace amounts in pink Spinels (Muhlmeister, et. al., 1993), and any magnetic attraction is due entirely to iron. Light pink Spinels have low iron content and are often diamagnetic, while darker pink Spinels tend to be weakly to moderately magnetic due to more iron. Three examples of pink Spinels ranging from diamagnetic to moderately magnetic are shown below. The diamagnetic gem on the left is colored entirely by chromium, while the weakly magnetic gem in the center has some some blue color due to iron. The moderately magnetic pink gem on the right has greater color intensity due to higher concentrations of both iron and chromium.
Pictured below is a remarkable example of two pink Spinels of similar color intensity that have strikingly different magnetic susceptibilities. The vivid pink gem on the left has weak magnetic susceptibility that is detectable with a magnetic wand, but so low that it cannot be measured with a Hoover balance. The vivid pink gem on the right is strongly magnetic, with a measured susceptibility that is higher than any pink Spinel in this study, and also higher than any red Spinel. The strong magnetism suggests the presence of cryptic iron (Fe3+), and this iron content is confirmed with a spectrometer. The gem on the left below shows UV fluorescence, while fluorescence is quenched by iron in the gem on the right.
Dark Pink Spinel
5.74ct., Moderate, SI 82
Vivid Pink Spinel
1.65ct., Strong, SI 165
Orange Spinel: Orange as the primary body color is an unusual color for Spinel. This color is also due primarily to traces of chromium. Why orange rather than pink is not known, but we can speculate it might be related to a charge transfer process and/or to additional traces of manganese, or perhaps traces of iron (Fe3+). Orange is usually a component color along with pink/red rather than a pure color in Spinel. All orange, pinkish orange and reddish orange Spinels we tested were either diamagnetic or very weakly magnetic, indicating very low levels of iron. None of our orange Spinels showed fluorescence, likely due to low levels of chromium. All three orange Spinels shown below are diamagnetic.
Orange Spinel, Burma
0.85ct., Diamagnetic, SI < 0
Reddish Orange Spinel
1.38ct., Diamagnetic, SI < 0
Pinkish Orange Spinel, Burma
0.77ct., Diamagnetic, SI <0
Purple, Color Change and Violet Spinels
Purple color and violet color in Spinel is primarily due to a mixing of blue color (derived from iron and cobalt) with pink & red color (derived from chromium). On average, the magnetic susceptibility of purple Spinel is intermediate between that of blue Spinel and pink/red Spinel. All purple Spinels tested were weakly magnetic (SI 22-56). Due to iron content, all showed magnetic attraction.
1.78ct., Weak, SI 30
Color change is an unusual phenomen in Spinel. The bluish gray Spinel pictured below shows pinkish purple color when incandescent light is applied. Chromium content within this gray gem is the cause of the color change. This gem also fluoresces red under long wave UV light. The low iron content in this weakly magnetic spinel permits fluorescence, which is likely due entirely to chromium.
Blue to Purple Color Change Spinel
3.10ct., Weak, SI 43
Daylight to Incandescent Light to UV Fluorescence
Violet is another unusual color for Spinel. The weakly magnetic gem below is colored by low levels of iron, cobalt and chromium. Purple gems contain higher concentrations of chromium than violet gems.
Completely colorless Spinel is rare. Two near-colorless Spinels were tested for this study. The Spinel with a slight tinge of pink due to chromium (below left) was diamagnetic, while the gem with a slight gray tinge due to ferrous iron (below right) was very weakly magnetic. Under long wave UV light, only the pinkish gem on the left contains enough chromium to show fluorescence.
Although not as widely used today as synthetic Corundum, synthetic Spinel is still one of the most common man-made gem materials, particularly blue synthetic Spinel. It appears frequently in class rings and costume jewelry. Blue and a few other colors of synthetic Spinel are mostly manufactured using the inexpensive flame-fusion method by which the Spinel develops around an elongated boule. These flame-fusion Spinels can be distinguished from natural Spinels by a slightly elevated refractive index due to a chemical variation involving higher aluminum content.
Blue synthetic Spinels of the flame-fusion type are typically colored only by cobalt. No iron is added, and therefore they are diamagnetic, another property that distinguishes blue flame-fusion Spinels from blue natural Spinels. When viewed under the Chelsea filter and also under long wave UV light, the light blue and medium blue gems pictured below left and center show a pale pink reaction due to low cobalt content, while the dark blue gem on the right appears bright red due to much higher cobalt content. None of the synthetic Spinels shown below contain enough cobalt to cause magnetic attraction.
Gray color is caused by ferrous iron (Fe2+). It is necessarily a component color rather than a stand-alone color in Spinel. Gray color is the result of combining dark blue color with light blue color (light transmission windows in the visible spectrum are at 430nm and 490nm). Our study found that Spinels which have gray as the dominant color component are weakly magnetic.
Pinkish Gray Spinel
2.11ct., Weak, SI 35
One anomalous blue synthetic flame-fusion Spinel was found to show a weak magnetic response due to manganese (Mn2+). This gem had an unusual bright "aqua" blue color, and fluoresced green under long wave UV light, indicating significant manganese content. With the aid of a spectrometer, we detected a small amount of manganese in other light blue synthetic Spinels, but only this one sample contained enough manganese to cause green fluorescence under long wave UV light, along with magnetic attraction. All light blue synthetic Spinels we tested fluoresced yellow under short wave UV light due to manganese. The purpose of adding manganese to synthetic Spinel is not clear, but perhaps it brightens the blue color by blending in a subtle yellow component. This could be similar to what we find in "neon" or "aqua" blue Paraiba Tourmalines that are naturally colored by copper (blue) and manganese.
Synthetic pink and red Spinels, like synthetic Ruby, are colored entirely by chromium, and both fluoresce red under UV light. The flux-grown pink synthetic Spinel shown below (left) is diamagnetic due to a low concentration of chromium. The gem fluoresces moderately red. The bright red flux-grown synthetic Spinel pictured below (right) contains enough chromium to induce a weak magnetic response. This gem is intensely fluorescent, far more than natural red Spinel. Synthetic pink and red Spinels cannot be definitively distinguished from natural pink and red Spinels by magnetic testing alone, although a moderate or strong magnetic response suggests iron content and natural origin.
Green and Yellow Synthetic Spinel: Most green synthetic Spinels show no magnetic attraction. The green flame-fusion synthetics shown below contain cobalt (Co2+, blue chromophore) as the primary chromophore in addition to a small amount of manganese (Mn2+, yellow chromophore) to create green color (yellow + blue = green). All 3 gems are diamagnetic. The green synthetic gem in the center mimics the color of rare natural blue-green Spinel, but a lack of magnetic attraction separates this synthetic gem from its natural counterpart. The left and center gems below flouresce pink under long wave UV light due to cobalt content. All 3 gems fluoresce yellow under short wave UV light due to manganese content.
Light Green Synthetic Spinel
0.95ct., Diamagnetic, SI <0
Dark Green Synthetic Spinel
6.11ct., Diamagnetic, SI <0
This concludes the section on color and magnetism in Spinel. For a list of the reading materials about Spinel referenced in this section, see the Spinel References section at the bottom of the Resources and Links page.
© Kirk Feral 2015, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website, without the expressed written consent of the copyright holder.
Medium Blue Synthetic Spinel
10.86ct., Diamagnetic, SI <0
This Anomalous Blue Synthetic Spinel
is Magnetic Due to Manganese
Yellow-Green Synthetic Spinel
6.32ct, Strong, SI 122
Fluorescence: Chromium causes some pink Spinels to show very strong fluoresce under long wave ultraviolet light. This occurs only in diamagnetic or weakly magnetic gems with low iron content. In dark red natural Spinels, UV fluorescence is weak or absent because most gems have higher iron content. As with Ruby, high chromium content along with low iron content are the conditions needed for maximum UV fluorescence, which appears as a red glow. Lower concentrations of chromium produce weaker fluorescence, and higher concentrations of iron mask fluorescence. The Spinels that showed the greatest fluorescence in our study were reddish pink or orangey pink gems from Burma and Tanzania. This includes the beautiful pinkish red Mahenge Spinel from Tanzania (below right). Vivid orangey-pinkish-red "Jedi" Spinels from Burma are also known to be highly fluorescent, but no samples were available for this study.
Pinkish Red Spinel, Burma
4.50ct., Weak, SI <20
Olive Green Synthetic Spinel
14.29ct, Diamagnetic, SI <0
The greenish yellow synthetic flame-fusion Spinel shown below (left) is colored only by manganese in relatively high concentration, and it shows a strong magnetic response due to the manganese. It also fluoresces bright yellow/green under UV light. The yellowish "olive" green synthetic flame-fusion Spinel on the right has a low concentration of manganese and shows a diamagnetic response. It fluoresces yellow only under short wave UV light. Yellowish-green and greenish-yellow colors found in synthetic Spinels doped with manganese are extremely rare among natural Spinels.
Fluorescence: Blue synthetic flame-fusion Spinels with unusually high manganese content fluoresce green under both long wave and short wave UV light. Blue flame-fusion synthetic Spinels with relatively high cobalt content fluoresce pink or red under long wave UV light due to cobalt, and fluoresce yellow/green under short wave UV light, presumably due to a minor amount of manganese. Because of the quenching effect of iron in natural Spinels, pink/red UV fluorescence from cobalt is difficult to detect, except in the rare Cobalt Spinel.
Green is rarely seen as the dominant body color in gem Spinel. Such green Spinels have significant blue or gray as the secondary color, and these gems can be described as blue-green and gray-green Spinels. The primary coloring agent is iron, and iron to iron (Fe2+-Fe3+) intervalence charge transfer is primarily responsible for the green color. The two rare Spinels pictured below show green as the dominant hue (light blue-green on the left and light gray-green on the right). Both gems are weakly magnetic.
0.96ct., Weak, SI 48
A variety of natural pure green Spinel called Chlorospinel is colored by ferric iron (Fe3+) in addition to copper (Cu2+). Chlorospinels are very rare as faceted gems, and none were available for this study.
Blue-green Synthetic Spinel
1.11ct., Diamagnetic, SI < 0
Mahenge Spinel, Tanzania
2.31ct., Weak, SI <20
Blue Spinel gems manufactured by the slower and more costly flux-grown method have a vivid blue color due to high cobalt content. They are generally small in size, faceted from small synthetic crystals that are shaped like natural octahedral Spinel crystals, as opposed to the large rounded boule shapes of flame-fusion Spinels. Because the major chemistry is the same as natural Spinel, the refractive index range is also the same. While the common flame-fusion synthetic blue Spinels are almost always diamagnetic, flux-grown blue Spinels are weakly magnetic due to their higher cobalt content. This is the only gem material of any kind that we have found that shows magnetic attraction entirely as a result of cobalt. No iron or manganese can be detected.
Blue flux-grown synthetic Spinel could be mistaken for natural Cobalt Spinel, as both have the same strong blue color, the same refractive index, and both show weak magnetic attraction. However a moderate to strong magnetic response in any vivid blue Spinel would indicate iron content and natural origin. Fortunately, flux-grown blue Spinels are rarely found as a gemstones because they are time-consuming to produce, costly, and small in size. Blue flux-grown Spinels do not fluoresce as brightly under long wave UV light as do dark blue flame-fusion synthetic Spinels, possibly because of concentration quenching produced by a higher concentration of cobalt.
Flux-grown Synthetic Blue Spinel Crystal
1.49ct., Weak, SI <20
1.51ct, Moderate, SI 82
405 nm (Cobalt) & 475 nm (Manganese)
One ight colored blue-green or “mint” green synthetic Spinel in our study (shown below) fluoresces two different colors under long wave UV light (LWUV), depending on wavelength. Pink fluorescence under near-ultra-violet light (405 nm) reveals the presence of cobalt, which imparts the blue color component of the mint color. Green fluorescence under true long wave UV light (375 nm) reveals manganese, which imparts the yellow color component (yellow + blue = green). Strong green fluorescence from manganese is also seen under short wave UV light. The moderately magnetic response of this gem is due entirely to manganese.
Mint Green Synthetic Spinel